1. FIELD OF THE INVENTION
The present invention relates to digital communication systems in which the waveform of the electromagnetic carrier signal is varied in accordance with the binary data being transmitted. More particularly, the present invention pertains to a peak to peak data communications scheme in which the distance between two successive peaks, one positive peak and one negative peak, represents the digital data transmitted.
2. DESCRIPTION OF THE PRIOR ART
Digital modulation is the process of transmitting base band digital information over a communications channel by altering certain aspects of a carrier signal. The carrier signal must be altered so that discrete states are created with very little chance for ambiguity between the states. These discrete states correspond to one or more binary bits of data and the resulting modulated carrier is more immune to noise than analog modulated carriers which represent continuous signal transitions with no discrete states.
To get the maximum amount of data over a given digital communications channel, small bandwidths and high bit rates are used. For example, more data can be transmitted over a frequency multiplexed communications channel, such as used in satellite links, when data communication techniques are used which have narrow bandwidth requirements per carrier while still providing high data rates.
Some carrier signals for digital modulation are based on sinusoidal waveforms since they require the least amount of bandwidth of any waveform. The process of altering the characteristics of the sinusoidal waveform for digital modulation is very important considering the high cost of communication channels in use at the present time. Heretofore, digital sinusoidal modulation has been accomplished by three basic means. These three classical forms of digital sinusoidal modulation are amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK) (see FIG. 6). In ASK, the amplitude of the carrier is varied or shifted in response to changes in the digital data. In FSK, the frequency of the carrier is varied or shifted in response to changes in the digital data. In PSK, the phase of the carrier is varied or shifted in response to changes in the digital data.
Each of these three forms of modulation has disadvantages. ASK is especially susceptible to atmospheric noise and fading. FSK requires the receiver to detect two discrete frequencies which takes time since several cycles of each frequency must be received before the frequency can be acquired and detected. PSK requires complex receiver circuitry to detect phase changes and elaborate filtering to control spurious outputs that reduce efficiency.
The three classical forms of sinusoidal digital modulation have one element in common. They use fixed time slots for varying the characteristics of the carrier. These fixed time slots cause the variations in the carrier to occur at random points along the sinusoidal waveform. When these changes to the carrier occur, spurious frequencies are generated that cause the modulation bandwidth to expand requiring complex filtering to reduce the amplitude of the spurious frequencies. As the bit rate increases; these variations in the carrier occur more frequently causing even more spurious output and making demodulation even more difficult.
U.S. Pat. No. 4,737,969 issued Apr. 12, 1988 to Steel et al discloses a digital communications scheme having relatively high bit rates and using digital pulses having a minimal frequency spectrum. U.S. Pat. No. 4,827,488 issued May 2, 1989 to Shibano discloses a digital communications scheme in which a sinusoidal carrier signal is demodulated in a PSK system by controlling the oscillator with a feedback signal derived from the received sinusoidal carrier.
U.S. Pat. No. 4,905,177 issued Feb. 27, 1990 to Weaver, Jr. et al discloses a method of phase modulation using direct digital synthesis circuits to create a phase modulated carrier.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.